This is a national stage application of PCT/KR97/00069 filed Apr. 29, 1997.
1. Field of the Invention
This invention relates to a method and composition for removing scale and rust (hereinafter referring to as simply xe2x80x98scalexe2x80x99) from a metal surface.
Particularly this invention relates to a method for efficiently removing the scale especially on stainless steel and other alloyed steel containing Ni and/or Cr without forming NOx gases, pickling solutions used for the method and a process for the regeneration of spent pickling solutions resulting from the method.
As is well known, stainless steel is widely used in various industrial applications due to its superiority in properties including corrosion resistance, oxidation resistance, and high temperature strength. Such stainless steels can be classified by its alloy composition into Cr-Stainless Steel (Cr: 11-27 wt %) and Ni-Stainless Steel (Cr: 15-28 wt %, Ni: 4-22 wt %). Further the Cr-stainless steel can be classified by its metallurgical structure into ferritic stainless steels and martensitic stainless steels, and the Ni-stainless steel can be classified into austenitic stainless steels and dual phase stainless steel having both ferrite phase and austenite phase.
Conventionally the stainless steel and other alloyed steel containing Cr and/or Ni are heattreated for high temperature oxidation after or during hot or cold rolling in production process or even on welding operation on finished products.
During the heat treatment, scales of oxides or carbides having a dense structure hard to be oxidized are formed on the surface of the stainless steel or the alloyed steel.
Since stainless steel contains Ni and Cr as essential components and Mo, Ti, Mn, Zr, Nb, N and other elements as additional components, the scales formed during the heat treatment consists mainly of Fe, Fe3O4, Fe2O3, NiO, Ni3O4, Ni2O3, CrO3 and Cr2O3.
As described above, since the scale has dense structures and is hard to be oxidized, it is hardly removed by a conventional pickling process.
Therefor to easily remove those scales, the stainless steel or other alloyed steels are typically pre-treated using salt bath, shot-blast or neutral salt electrolysis in chemical, physical or electrical way, respectively, before pickling.
By the pre-treatment, the scales on the surface can be partially removed to a certain extent and then completely removed by dipping in a conventional pickling solution.
However each of the pre-treatment processes using shot-blast, salt bath or neutral salt electrolysis has several problems as described below:
The pre-treatment using shot-blast is brought about by shotting spheroidal particulates of metal or glass onto the surface of metal under high pressure to physically remove the scale formed on substrate to increase the effect of pickling process to be followed.
However such pre-treatment using shot-blast may degrade the surface roughness of substrate and may cause dust formation.
The pre-treatment using salt bath is brought about by dipping the substrates on which the scales are formed into the salt bath containing sodium hydroxide, sodium nitrate, and sodium chloride in a temperature range from 480 to 520xc2x0 C. to improve the pickling efficiency by softening the scale structure.
However using the salt bath may produce Cr+6 which may give rise to environmental problems and requires increased energy consumption.
The pre-treatment using neutral salt electrolysis is brought about by applying electric power to the solution containing neutral salt such as sodium sulfate to form bubbles to improve the pickling efficiency by softening the scale structure with the formed bubble.
However the use of this neutral salt also has the disadvantage that energy consumptions are increased.
In addition to the above-described disadvantages of each pre-treatment method, pre-treatment itself increases the length of the process.
After conducting such pre-treatment as described above, the substrate covered with scales are treated with pickling solution in order to obtain completely bared surfaces.
Conventionally aqueous blends containing hydrofluoric acid and nitric acid are used as pickling solutions. Optionally different classes of compounds including hydrochloric acid and sulfuric acid can be added to the pickling solution.
The components and composition ratios of the pickling solution can be varied depending on the type of substrate and end use.
During the pickling process using the mixed acid substantially of hydrofluoric acid and nitric acid, the metal ions formed on the substrates are believed to react with the mixed acid as follows;
Fe0+6H++6NO3xe2x88x92xe2x86x92Fe3++3NO2↑+3H2O+3NO3xe2x88x92xe2x80x83xe2x80x83(1-1) 
Ni0+6H++6NO3xe2x88x92xe2x86x92Ni3++3NO2↑+3H2O+3NO3xe2x88x92xe2x80x83xe2x80x83(1-2) 
Cr0+6H++6Fxe2x88x92xe2x86x92Cr+3+3HF+3/2H2↑+Fexe2x88x92xe2x80x83xe2x80x83(1-3) 
Fe+3+nHFxe2x86x92FeFn(3xe2x88x92n)+nH++nFxe2x88x92xe2x80x83xe2x80x83(1-4) 
As shown in the above, when pickling process is carried out in conventional manners, noxious and corrosive hydrohalide acid fumes may be liberated into the atmosphere to cause environmental contaminations.
Specifically the nitric acid reacts with the metal ions of the scale to form the noxious NOx gases representing nitrogen oxide and the NOx gases formed during the pickling process are released into the atmosphere, while unreacted nitric acid and volatile hydrofluoric acid are simultaneously volatilized into the air to give rise to additional environmental problems. (HF: Specific gravity 0.987, Boiling Point 19.4xc2x0 C., Melting Point xe2x88x9292.3xc2x0 C., HNO3: Specific gravity 1.502, Boiling Point 86xc2x0 C., Melting Point xe2x88x9242xc2x0 C.).
Therefore there has been a need for a method of pickling and a pickling solution to avoid the disadvantages inherent in the conventional pickling method and solution, and preferably a method of pickling and a pickling solution in which any pre-treatment process could be abbreviated.
U.S. Pat. No. 4,572,743 discloses a method for pickling metallic surfaces especially unalloyed steel surfaces prior to further treatment by phosphating and varnishing in order not only to decrease the workpiece but also to remove rust and scale formed thereon.
The ""743 reaches to use non-aqueous pickling system based on organic solvent, especially to use homogeneous organic pickling solutions based on low-boiling halogenated hydrocarbons as the basic component to avoid the disadvantages of aqueous pickling system.
U.S. Pat. No. 3,936,316 discloses a process and composition for pickling metals especially iron based metals prior to metal finishing. The ""316 teaches to use the hydrohalide pickling solution containing urea in order to reduce or completely eliminates the excessive liberation of noxious and corrosive hydrohalide acid fumes.
Non of these references did not disclose or suggest any pickling method or solutions which could fundamentally eliminate the problems inherent in the conventional pickling method and solution such as liberation of noxious gases, cost increase, low efficiency of pickling process and necessity of pre-treatment.
A need therefore exists for an improved pickling method and solution which could fundamentally avoid the disadvantages of conventional pickling systems, preferably together with an improved process for regenerating and recycling the spent pickling solutions which could reduces the cost of pickling operation.
One object of the invention is to provide a generally improved method for pickling iron based metal alloys containing Cr and/or Ni, which is capable of obviating the problems in the prior art.
Another object of the invention is to provide a method which is capable of removing the scales of oxides or carbides without forming noxious gases which is injury to personnel and which does not impair the efficiency of the pickling process itself.
A further object of the invention is to provide a method for pickling metal without conducting pre-treatments before pickling process.
Another object of the invention is to provide a method for pickling metal which could reduce the cost of pickling operation and increase the process efficiency.
A further object of this invention is to provide a pickling solution which can be used for removing the scale formed on the metal, which do not cause the formation of noxious gases injurious to personnel.
Another object of the invention is to provide a pickling solution which can be use for efficiently removing the scale formed on the metal without conducting pre-treatments before pickling process.
And a further object of this invention is to provide a process for efficiently regenerating the resultant spent pickling solution after pickling process.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
The invention in the first aspect discloses a method for pickling a metal surface to remove the scale formed thereon, comprising contacting the metal surface with a pickling solution comprising, as essential components, nitrate and fluorides, and, as optional components, at least one selected from the group consisting of sulfuric acid, sulfates, phosphoric acid, phosphates, peroxides, amines and amides, nitric acid, sulfonic acid and sulfonates.
In this aspect, the metal surface on which scales are formed is contacted with the pickling solution containing not mixed acids such as hydrofluoric acid and nitric acid but nitrates and fluorides, and thereby not only noxious or toxic gases including NOx gases which represents nitric oxides and hydrohalide acid fumes are not generated but also the efficiency of the pickling can be improved.
Further this method can abbreviate the pretreatment conventionally used in prior art with maintaining or even improving the pickling efficiency, thereby the line of process may be reduced.
The invention in the second aspect discloses a pickling composition useful for the method for pickling a metal surface according to the invention wherein the pickling solution comprising an effective pickling amount of a mixture of nitrate and fluorides, as essential components; and optionally at least one selected from the group consisting of sulfuric acid, sulfates, phosphoric acid, phosphates, peroxides, amines and amides, nitric acid, sulfonic acid and sulfonates.
Further according to the third aspect of this invention there provided a process for efficiently regenerating spent pickling solution after pickling, which comprises; contacting a metal surface with the pickling solution containing nitrates and fluorides to remove the scale formed on the metal surface; filtering the resultant spent pickling solution to separate liquid phase containing metal ions dissolved during pickling and solid phase mainly composed of oxidized steel and sludge, and then recovering the solid phase; cooling the filtrates to deposit the dissolved metal ion as metal salt and then recovering the metal salt; and recycling the resultant regenerated pickling solution for use in further process.
Further according to the forth aspect of this invention, there provided a process for efficiently regenerating spent pickling solution after pickling, which comprises;
contacting a metal surface with the pickling solution containing nitrate and fluorides to remove the scale formed on the metal surface; filtering the resultant spent pickling solution to separate liquid phase containing metal ions dissolved during pickling and solid phase mainly composed of oxidized steel and sludge, and then recovering the solid phase; passing the filtrates into ion exchange resin tube to absorb the metal ion; and recycling the resultant regenerated pickling solution for use in further process.
According to the process for regenerating spent pickling solution of this invention, the spent pickling solution containing metal dissolved during pickling process can be efficiently regenerated with maintaining the metal ion concentration below a predetermined level and additionally iron oxides and metal salt can be attained as by-products.